The number of echoes combined into a single image or image set in multiple echo imaging sequences or echo-train techniques such as rapid acquisition with relaxation enhancement (RARE), fast-spin echo (FSE), and turbo spin-echo (TSE). In RARE imaging, the ETL typically equals the acceleration factor.
As for tooth pain, it is caused by a reaction of the nerves inside a tooth's pulp chamber with the severity dependent upon the type and degree of the stimulus. What follows are some examples of symptoms you may be feeling and their possible causes.
Pain that is localized around a particular tooth; Pain that radiates to the jaw or ears; Pain that radiates to the shoulders, head, or neck; Increased sensitivity to heat or cold; Toothache that worsens when chewing; A tooth or gums that is tender to the touch; Pain that is aggravated by movement or exercise; Tooth pain that prevents you from sleeping